Various methods have been employed to control the velocity of a motor, including motors used in an image forming apparatus such as a scanner, printer, or electrophotographic copying apparatus. Such image forming apparatus require precision control of the motor velocity to form a quality image. For example, when printing an image, precise control of the relative velocity (in the printing direction) between the printer and the media on which the image is being formed is required. Since the relative velocity is generally low, precision control of motors operating at a slow speed may be preferable to reduce/eliminate the need for potentially expensive mechanical speed reduction suitable to meet the velocity regulation requirements.
Stepper motors have been employed, but have significant limitations to their performance. To overcome some of these limitations, a Phase Lock Loop (PLL) motor speed servo controller with an incremental optical encoder for position feedback can be employed. Use of PLL controllers for precision motor speed control is known to those skilled in the art. For example, U.S. Pat. No. 5,212,434 describes a speed controller utilizing a phase-locked stepping servomechanism. A reference by D. H. Smithgall titled "A Phase-Locked Loop Motor Control System", IEEE, November 1975, describes the use of an analog circuit to measure phase error between a stable time reference and the signature from an encoder coupled to the motor.
A reference by R. E. Holm titled "Multifunction Chip Steers Hard Disk Drives", Electronic System Design Magazine, April 1988, describes a motor control method using a microcontroller to measure motor velocity by using a counter and a higher frequency clock to make time period measurements of an encoder. That is, the controller makes precision time measurements for when transitions of the encoder signal occur, compares the time measurements to a stable time reference, and provides a torque correction signal which is derived from a variation of a PID (Proportional plus Integral plus Derivative) calculation on the error. The motor speed is regulated by the calculated error. The quality and resolution of the encoder is critical if this method is employed. The encoder requires incremental position feedback sufficiently frequent to support the servo controller requirements to regulate velocity and reject disturbances that might cause velocity variations.
While such apparatus may have achieved certain degrees of success in their particular applications, current methods of motor control do not optimize the use of encoder information.
Accordingly, a need continues to exist for a method for obtaining more and better information from an optical encoder for controlling the speed of a motor.